In many applications where a load is driven by an electric motor, it is desirable to vary the speed of the motor to provide optimum performance of the driven load.
DC motors have long been widely used for this purpose, since the speed of a DC motor can be easily controlled by changing the voltage applied to its terminals. A good measure of the DC motor speed is the armature voltage, and this voltage is used in many control systems for feedback purposes to regulate the motor speed. Similarly, armature current is a good indication of load torque.
However, a DC motor is expensive because of the complex windings on its armature, and because a commutator is required. In addition, a DC motor has brushes to conduct current to the armature. These brushes have a limited life, making periodic maintenance necessary.
A "squirrel cage" AC induction motor, on the other hand, is relatively inexpensive, since the rotor windings consist of metal bars which are cast into steel laminations that make up the remainder of the rotor and the stator windings can easily be inserted in slots in stator laminations. An induction motor, at least in a polyphase variety, has no brushes, no moving parts other than the rotor, and virtually no maintenance.
While the speed of an induction motor can be varied to a certain extent by varying the voltage applied to the stator winding, and holding the applied frequency constant, only a limited speed range is available. And the electrical losses in the motor can be excessive, if the load is too great, with a serious potential for burning out the motor.
It is, however, possible to efficiently control the speed of an induction motor by varying the frequency as well as the magnitude of the AC voltage applied to the motor.
Solid state inverters are often used for this purpose. The inverter generates variable frequency multi-phase AC voltages to feed the induction motor stator windings and varies the magnitude of these voltages to be essentially proportional to the frequency. The waveform of the applied voltage may be a square wave; a multi-stepped rectangular waveform that approximates a sine wave; or a true sine wave, commonly generated by pulse-width modulation. For any of these waveforms, AC power in AC power mains is rectified to provide a DC voltage on a DC bus, and solid state switches, connected between the bus and the motor windings, operate sequentially to provide the AC voltages to the motor stator windings.
When operating at no load, the induction motor rotor operates at a speed that is essentially proportional to the applied frequency, known as the "synchronous speed", since it is the same speed that a synchronous AC motor will run at the same applied frequency. When a load is applied to an AC induction motor, the rotor speed changes in an amount known as "slip", with the rotor speed being slower than synchronous speed for loads that absorb power, and higher than synchronous speed for loads that attempt to drive the motor at a speed greater than unloaded speed. The difference between synchronous speed and rotor speed is "slip speed".
When the rotor speed is greater than synchronous speed, the induction motor becomes a generator and returns power to the source, which in this case is the DC bus, derived from the AC mains. If no mechanism is available for the DC bus to absorb this "regenerative" energy, the bus voltage rises, potentially to a destructive level.
Regeneration may result from as simple an action as turning the speed control of an inverter to a lower level or attempting to connect a coasting motor to an inverter set to a lower frequency. In the prior art, many methods have been devised to absorb any regenerative energy that may appear, or to shut down the inverter if regeneration is attempted.
Whatever means is used to absorb regenerative energy, it adds significantly to the cost of the inverter, since instantaneous power may be quite substantial. If the inverter is programmed to disable itself by "tripping" or shutting down on regeneration, there may be many nuisance shut downs. When the application does not require regeneration, it is therefore desirable for an inverter to be able to detect when regeneration is imminent and to prevent it at a signal level where the cost is negligible, and without shut down or other disruption.
One object of this present invention therefore is to provide an AC induction motor control that senses when the rotor is running faster than desired, and prevents regeneration, thereby saving the circuitry cost and complexity required to absorb this regenerative energy, and avoiding the inconvenience of resetting an inverter that has tripped out because regeneration has occurred.
In prior art, the speed of the induction motor has been controlled by applying variable frequency voltages, the frequency of which approximates the desired speed and the magnitude of which is proportional to the frequency. At very low speeds, the voltage is usually "boosted" to account for the impedance change of the motor at very low frequencies. At any given speed setting, the frequency remains constant, and therefore the rotor speed will vary with changing loads according to the slip of the rotor.
However, if at any given setting, the load on the motor becomes excessive, the slip will increase substantially beyond the rated level, and the motor current will become correspondingly excessive or even destructive. For this reason, most prior art inverters are designed to "trip" out during overload conditions, requiring resetting and restarting. In most cases, the motor must coast to a stop before restarting.
Unlike the DC motor, the operating speed of the AC induction motor is not easily discernible by observing voltages and currents. For example, the voltages and currents of the induction motor are AC signals, and as such, if used directly, require rectification and filtering to determine the absolute values for feedback purposes. At lower speeds, however, with frequencies of only a few Hertz, the filtering required to obtain a DC signal has such a long time constant as to render any control action very sluggish.
And, while the torque of a DC motor is almost exactly proportional to the armature current, this is not true in an induction motor, where the relationship between torque and input current is very non linear. Some AC induction motors require almost as much current at no load as they do at full load.
In the prior art, speed measuring devices, such as tachometer generators, have been used to improve speed regulation by providing a speed feedback signal to the motor control, and the input frequency is adjusted accordingly to compensate for the slip. Various methods, known generally as "vector control", are used to adjust the input voltage or current to a level appropriate for the existing slip.
The use of speed measuring devices, while effective, requires access to the motor shaft, and additional connections between the motor and the control, both of which add to the cost of a system. When it is desired to add speed control to an existing motor installation, it may not be practical or even possible to add the speed measuring device.
It is, therefore, a second object of the present invention to employ electronic measurement and signal conditioning to interpret the information available at the terminals of an AC induction motor to determine motor load and speed conditions without excessive filtering, and to use this information as feedback in a closed loop speed control for a power supply, such as an inverter.
More specifically, it is an object of this invention to use the voltage and current of the induction motor stator windings, with improved signal conditioning, to determine speed and load and, using this information as feedback in the motor control, to accurately regulate the speed of the motor under varying conditions of load.
A third object of the invention is to use the load information so obtained to provide safe operation during overload conditions in the motor by reducing speed, and to allow starting a motor that is rotating prior to the time it is started without creating excessive currents or torques.